High strength cold rolled steel plate and method for production thereof

ABSTRACT

The present invention provides a high strength cold-rolled steel sheet, consisting essentially of, by mass %, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balance being iron and inevitable impurities, and having a structure which substantially comprises ferrite phase and martensite phase. Since the high strength cold-rolled steel sheet of the invention has ductility in which an elongation is 18% or more, excellent stretch-flangeability in which a hole-expanding ratio is 60% or more and a tensile strength of 780 MPa or more, it is favorable for use in a structural member of automobile.

TECHNICAL FIELD

[0001] The present invention relates to a high strength cold-rolledsteel sheet, favorable for use in a structural member of machine,particularly in a structural member of automobile, which has a tensilestrength of 780 MPa or more, and a manufacturing method thereof.

BACKGROUND ART

[0002] From the point of view of achieving weight reduction ofautomobile for the purpose of reduction in fuel consumption and ensuringsafety for occupants of automobile, application of a high strengthcold-rolled steel sheet having a tensile strength of 780 MPa or more toa structural member of automobile has been studied. However, since sucha high strength cold-rolled steel sheet as described above is inferiorin ductility and stretch-flangeability to a mild cold-rolled steelsheet, it is difficult to subject the high strength cold-rolled steelsheet to press-forming. The term “stretch-flangeability” as used hereinmeans a property resisting to generation of cracks on a blank end faceof steel sheet when it is press-formed and is evaluated, based on ahole-expanding ratio measured by means of hole-expanding test defined bythe Japan Iron and Steel Federation Standard: JFST 1001-1996.

[0003] To date, various methods for improving stretch-flangeability of ahigh strength cold-rolled steel sheet have been disclosed as describedbelow.

[0004] In JP-B No. 7-59726, JP-A Nos. 2001-226741, 10-60593 and9-263838, high strength cold-rolled steel sheets which have each aimedfor improving stretch-flangeability by controlling structure throughoptimizing steel compositions and manufacturing conditions, andmanufacturing methods thereof are disclosed. More specifically, forexample, in JP-A No. 9-263838, a cold-rolled steel sheet is slowlycooled from soaking temperature at the time of annealing to allow secondphase to be uniformly dispersed in ferrite phase and, then, bainitephase is allowed to be uniformly dispersed in the ferrite phase as amain component by adjusting cooling rate and overaging temperature,thereby aiming for enhancing strength and improvingstretch-flangeability.

[0005] In JP-A No. 2001-355044, a high strength cold-rolled steel sheetin which ferrite phase is allowed to have higher strength and from 2% to20% of residual austenite phase is formed in the ferrite phase to aimfor simultaneously achieving strength enhancement andstretch-flangeability improvement is disclosed.

[0006] In JP-A No. 11-350038, a method for producing a complex phasetype high strength cold-rolled steel sheet which is excellent inductility and stretch-flangeability and has a tensile strength of about980 MPa by controlling compositions and producing conditions isdisclosed.

[0007] In JP-A No. 9-41040, a method for manufacturing a high strengthcold-rolled steel sheet which is excellent in stretch-flangeability bysubjecting a cold-rolled steel sheet to annealing in an α+γ region,cooling the resultant steel sheet by holding it in a temperature rangeof from 650° C. to temperature to stop pearlite transformation for 10seconds or more and, then, cooling the cooled steel sheet by holding itin a temperature range of from temperature to stop pearlitetransformation to 450° C. for 5 seconds or less is disclosed.

[0008] Further, prior arts as described below in regard to a highstrength cold-rolled steel sheet which, though not referring tostretch-flangeability, aims for enhancement of formability and the likeare also disclosed.

[0009] In JP-B No. 58-55219 and Japanese Patent No. 2545316, a methodfor producing a high strength cold-rolled steel sheet by more strictlydefining compositions and performing annealing under specifiedcontinuous annealing conditions is disclosed.

[0010] In JP-B No. 7-68583, a method for manufacturing a dual phase typehigh strength cold-rolled steel sheet which is excellent in mechanicalcharacteristics, spot-weldability and phosphatability by specifyingcontent of C, Si, and Mn, reheating conditions before hot rolling,soaking conditions, atmosphere and the like in continuous annealingafter cold rolling is disclosed.

[0011] In JP-B No. 8-30212, a method for manufacturing a high strengthcold-rolled steel sheet having high ductility and excellent bendingproperty by allowing structure after hot rolling to be uniformly finersuch that band structure is not generated therein and, then, allowingthe resultant structure after continuous annealing to be that in whichferrite phase and martensite phase are uniformly distributed isdisclosed.

[0012] In JP-B No. 5-57332, a method for producing a high strengthcold-rolled steel sheet which has a yield ratio of 0.65% or less and isexcellent in both surface property and bending property by heating steelcontaining Si and a comparatively large amount of Mn to austenite singlephase zone which is higher than Ac3 transformation temperature and,then, allowing complex phase structure comprising ferrite phase andsecond phase such as martensite phase to be formed in a cooling step isdisclosed.

[0013] In JP-B Nos. 1-35051 and 1-35052, a method for manufacturing ahigh strength cold-rolled steel sheet which is excellent in ductility bycontrolling heating temperature in continuous annealing, water-quenchingstart temperature, and overaging treatment temperature is disclosed.

[0014] In JP-B Nos. 7-74412 and 3-68927, a method for producing a highstrength cold-rolled steel sheet which is excellent in bending property,deep drawability, and resistance to seasoned crack by allowingcondensation of C to be low to thereby set austenite phase to be 5% orless by means of performing annealing in a high temperature range aftercold rolling is disclosed.

[0015] However, such conventional prior arts as described above haveproblems as described below.

[0016] In JP-B 7-59726, it is essential to perform averaging treatmentat such a high temperature as 350° C. or more and, in order tocompensate decrease of tensile strength to be caused by such hightemperature averaging treatment, a large amount of C which is areinforcing element has been added (in steel Nos. 9, 10, and 13according to the invention in Table 1, in order to have a tensilestrength of 980 MPa or more, 0.17% or more of C has been added.). Forthis reason, when the steel is spot-welded at the time of assembling anautomobile, tenacity of spot-welded portion is deteriorated and, as aresult, joint strength thereof is decreased. Further, since averagingtreatment temperature is high, energy cost in production is increased,thereby deteriorating productivity. Still further, when the steel has atensile strength of 980 MPa or more, a hole-expanding ratio is as low as56% (steel 9 according to the invention in Table 1), thereby allowingstretch-flangeability to be insufficient.

[0017] In JP-A No. 2001-226741, it is essential to perform austemperingtreatment after soaking in continuous annealing in order to generatebainite phase, but there is a problem in that consistent characteristicsof steel sheet can not be obtained in this treatment.

[0018] In JP-A No. 2001-355044, since residual austenite phase isallowed to exist, it is essential to generate bainite phase, therebydecreasing strength. The tensile strength shown in an example is as lowas from 600 MPa to 800MPa, thereby being incapable of consistentlyobtaining a tensile strength of 780 MPa or more. In order to enhancestrength, it is necessary to add a large amount of C, Si, and Mn,thereby inviting deterioration of weldability and the like.

[0019] In JP-A No. 11-350038, since an amount of C is as large as from0.10% to 0.15%, thereby deteriorating stretch-flangeability or tenacityof spot-welded portion.

[0020] In JP-A Nos. 9-41040 and 9-263838, since structure comprisesferrite phase and pearlite phase, or ferrite phase and bainite phase,tensile strength is as low as from 400 MPa to 700 MPa.

[0021] In JP-A No. 10-60593,-JP-B Nos. 58-55219 and 7-68583, andJapanese Patent No. 2545316, tensile strength of from 400 MPa to 700 MPacan only be obtained.

[0022] In JP-B Nos. 1-35051, 1-35052, 3-68927, 8-30212,5-57332 and7-74412, consistent and excellent stretch-flangeability can not beobtained.

DISCLOSURE OF THE INVENTION

[0023] An object of the present invention is to provide a high strengthcold-rolled steel sheet having an elongation of 18% or more, ahole-expanding ratio of 60% or more, and a tensile strength of 780 MPaor more and a manufacturing method thereof.

[0024] This object can be achieved by a high strength cold-rolled steelsheet consisting essentially of, in terms of percentages by mass, 0.04to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or lessS, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balance being ironand inevitable impurities and having a structure substantiallycomprising ferrite phase and martensite phase.

[0025] Further, this high strength cold-rolled steel sheet can berealized by a method for manufacturing a high strength cold-rolled steelsheet comprising the steps of: producing a steel sheet by hot rolling asteel slab having the aforementioned compositions, followed by coldrolling; heating the cold-rolled steel sheet at from 750° C. to 870° C.for 10 seconds or more; cooling the heated steel sheet down to from 550°C. to 750° C.; and cooling the cooled steel sheet down to 300° C. orless at a cooling rate of over 100° C./sec.

EMBODIMENTS OF THE INVENTION

[0026] In a high strength cold-rolled steel sheet having a tensilestrength of 780 MPa or more, it is necessary to allow structure tosubstantially be a dual-phase structure of ferrite phase and martensitephase. For such necessity, as described above, it is necessary toincrease an amount of C, thereby deteriorating stretch-flangeability,spot-weldability and, further, phosphatability.

[0027] The present inventors have studied on a steel sheet which, eventhough an amount of C is reduced, has a tensile strength of 780 MPa ormore and, further, excellent ductility in which an elongation is 18% ormore, and excellent stretch-flangeability in which a hole-expandingratio is 60% or more, and found that the steel sheet can be realized bya steel sheet consisting essentially of, in terms of percentages bymass, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P,0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balancebeing iron and inevitable impurities and having a structuresubstantially comprising ferrite phase and martensite phase.

[0028] Hereinafter, the present invention will be described in detail.

1) Compositions

[0029] C: C is an important element for giving a great influence ontensile strength, and reinforcing martensite phase which is generated atquenching. When an amount of C is less than 0.04%, a tensile strength of780 MPa or more can not be obtained, while, when it is over 0.10%,stretch-flangeability and spot-weldability are remarkably deteriorated.Accordingly, the amount of C is set to be 0.04 to 0.10%. Further, inorder to obtain a tensile strength of from 780 MPa to less than 980 MPawithout deteriorating stretch-flangeability or spot-weldability, it ispreferable to set the amount of C to be 0.04% to less than 0.070% and,further, in order to obtain a tensile strength of from 980 MPa to lessthan 1180 MPa, it is preferable to set the amount of C to be 0.070 to0.10%.

[0030] Si: Si is effective in enhancing ductility of dual phase typesteel sheet comprising ferrite phase and martensite phase. When anamount of Si is less than 0.5%, effectiveness thereof becomesinsufficient, while, when it is over 1.5%, a large amount of Si oxide isformed on a surface of steel sheet in a hot rolling step, therebygenerating surface defects. Accordingly, the amount of Si is set to be0.5 to 1.5%. Further, from the point of view of phosphatability, theamount of Si is desirably set to be 1.0% or less.

[0031] Mn:.Mn is an important element for suppressing generation offerrite phase in a cooling step of continuous annealing. When an amountof Mn is less than 1.8%, effectiveness thereof becomes insufficient,while, when it is over 3%, a slab crack is generated at the time ofcontinuous casting. Accordingly, the amount of Mn is set to be 1.8 to3%. Further, in order to consistently produce the steel sheet in acontinuous annealing step, the amount of Mn is desirably set to be 2.0to 2.5%.

[0032] P: when an amount of P is over 0.02%, spot-weldability isremarkably deteriorated and, accordingly, the amount of P is set to be0.02% or less.

[0033] S: when an amount of S is over 0.01%, spot-weldability isremarkably deteriorated and, accordingly, the amount of S is set to be0.01% or less.

[0034] Sol. Al: Al is added for performing deoxidization of steel orprecipitating N as AlN. When an amount of Sol. Al is less than 0.01%,the deoxidization or the precipitation of AlN is not sufficientlyperformed, while, when it is over 0.1%, effectiveness thereof issaturated, thereby inviting a cost increase. Accordingly, the amount ofSol. Al is set to be 0.01 to 0.1%.

[0035] N: since N deteriorates formability of steel sheet, an amount ofN is desirably as low as possible. However, when the amount thereof isreduced more than necessary, a refining cost is increased. Accordingly,the amount of N is set to be 0.005% or less such that it does notsubstantially deteriorate the formability.

[0036] Besides the aforementioned elements, when at least one elementselected from 0.01 to 1.0% Cr, 0.01 to 0.5% Mo, 0.0001% to 0.0020% B,0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001% to 0.05% V, and 0.001 to0.05% Zr is allowed to be contained, there is an advantage in thatstructure adjustment at the time of continuous annealing is facilitated,or stretch-flangeability is enhanced by suppressing an occurrence inwhich a carbide or a nitride is formed in the steel sheet during castingor in a hot rolling step and, then, crystal grains come to be coarse.When a content of each element is less than the lower limit, theaforementioned effects are not sufficiently performed, while it is overthe upper limit, the ductility is liable to be deteriorated.

2) Structure

[0037] Structure of steel sheet substantially comprises two phases of:ferrite phase and martensite phase. Besides these two phases, bainitephase in which iron is a main constitutional element or austenite phasemay not deteriorate effectiveness of the present invention, so long asit is contained in an amount of less than 2% in terms of volumefraction. Further, compounds containing iron such as cementite may becontained in the ferrite phase, the martensite phase or an interfacebetween ferrite and martensite phases. Still further, compounds such asAlN and MnS may not impair the effectiveness of the present invention,so long as each of the composition elements or impurity elements iswithin the scope of the invention.

[0038] When a volume fraction of martensite phase is 30 to 45%, in therange of from 780 MPa to less than 980 MPa of tensile strength, or whenit is 45 to 60%, in the range of from 980 MPa to 1180 MPa of tensilestrength, more excellent stretch-flangeability can be obtained.

[0039] Further, in the range in which desired strength can be achieved,a tempering treatment can appropriately be performed on the martensitephase.

3) manufacturing method

[0040] Firstly, a slab having the aforementioned compositions isproduced by continuous casting method or ingot making plus bloomingmethod and, then, either after reheating or directly, the resultant slabis hot-rolled. A final rolling temperature (finishing temperature) athot rolling is desirably from Ar3 transformation temperature to 870° C.,in order to allow structure to be finer to thereby enhance ductility orstretch-flangeability. The hot-rolled steel sheet is cooled and, then,coiled. A coiling temperature is desirably 620° C. or less for thepurpose of enhancing ductility or stretch-flangeability.

[0041] Next, the resultant steel sheet is cold-rolled to be in a desiredthickness. At this time, a cold-rolling reduction rate is desirably 55%or more for the purpose of enhancing ductility or stretch-flangeabilityby allowing structure to be finer.

[0042] Finally, the cold-rolled steel sheet is annealed under conditionsas described below in a continuous annealing furnace.

[0043] i) Heating: from 750° C. to 870° C. for 10 Seconds or More

[0044] When a heating temperature is less than 750° C., a sufficientamount of austenite phase is not generated and, accordingly, highstrength can not be aimed for, while, when it is over 870° C.,transformation into austenite single phase occurs allowing structure tobe coarse, thereby deteriorating ductility or stretch-flangeability.Further, when a heating time is less than 10 seconds, austenite phase isnot sufficiently generated and, accordingly, high strength can not beaimed for.

[0045] ii) Primary Cooling (slow cooling); Cooling Terminal Temperature:from 550° C. to 750° C.

[0046] When a cooling terminal temperature is less than 550° C, a volumefraction of ferrite phase becomes unduly high, strength becomesinsufficient, while, when it is over 750° C., not only ductility isdeteriorated by subsequent rapid cooling, but also flatness of steelsheet is deteriorated. A cooling rate at this time is desirably set tobe 20° C. in the range of from 550° C. to 750° C. depending oncompositions such that a volume fraction of austenite phase can beadjusted to be from 30% to 45% or from 45% to 60%, namely, a volumefraction of martensite phase can ultimately be adjusted to be from 30%to 45% or from 45% to 60%.

[0047] iii) Secondary Cooling (rapid cooling); Cooling Rate: Over 100°C/sec; Cooling Terminal Temperature: 300° C. or less

[0048] When a cooling rate is 100° C./sec. or less, quenching becomesinsufficient and, accordingly, high strength can not be aimed for. Inorder to consistently aim for high strength, rapid cooling is desirablyperformed at a cooling rate of 500° C./sec or more. Further, when acooling terminal temperature is over 300° C., either bainite phase isgenerated, or austenite phase remains, thereby deterioratingstretch-flangeability. In order to obtain consistent excellentstretch-flangeability, the cooling terminal temperature is preferablyset to be 100° C. or less.

[0049] After the rapid cooling, the resultant steel sheet may be held atthe cooling terminal temperature for from 5 minutes to 20 minutes orsubjected to tempering treatment at from 150° C. to 390° C. for from 5minutes to 20 minutes. By performing the tempering treatment, themartensite phase which has been generated at the rapid cooling istempered, thereby enhancing ductility and stretch-flangeability.Further, when a tempering temperature is less than 150° C., or atempering time is less than 5 minutes, such effect as described abovecan not sufficiently be obtained. On the other hand, when the temperingtemperature is over 390° C., or the tempering time is over 20 minutes,strength is remarkably decreased, thereby being sometimes incapable ofobtaining a tensile strength of 780 MPa or more.

[0050] Still further, it is preferable that the obtained steel sheet issubjected to temper rolling at a reduction rate of from 0.1% to 0.7% tothereby eliminate yield elongation completely.

[0051] Furthermore, the steel sheet according to the present inventioncan be electroplated, hot-dip galvanized or applied with solidlubricant.

Example 1

[0052] Steel Nos. 1 to 10 having respective compositions as shown inTable 1 were each cast into slab. The cast slab was reheated at 1250°C., hot-rolled at a finishing temperature of about 860° C., and slowlycooled at about 20° C. to produce a hot-rolled steel sheet having athickness of 2.8 mm by simulating coiling at 600° C. for one hour. Next,the hot-rolled steel sheet was cold-rolled to-produce a cold-rolledsteel sheet having a thickness of 1.2 mm and, then, the cold-rolledsteel sheet was subjected to heating treatment which simulatedcontinuous annealing. The continuous annealing was performed underconditions that a temperature of the steel sheet was elevated at aheating rate of about 20° C. and, then, the steel sheet was soaked at830° C. for 300 seconds, slowly cooled down to 700° C. at about 10°C.sec and, thereafter, rapidly cooled in jet-flowing water having atemperature of 20° C. A cooling rate of such rapid cooling was about2000° C/sec. Finally, the steel sheet was subjected to temperingtreatment at 300° C. for 15 minutes, cooled and, then, subjected totemper rolling of 0.3% to produce steel sheet Nos. 1 to 10. Thereafter,in regard to the steel sheet Nos. 1 to 10, respective tensilecharacteristics and hole-expanding ratios (λ) were measured.

[0053] In regard to the tensile characteristics, a JIS No. 5 test piece(JIS Z 2201) was obtained along each of a rolling direction and adirection at a right angle thereto and subjected to a test in accordancewith JIS Z 2241 to determine yield strength (YP), tensile strength (TS),and elongation (El).

[0054] In regard to the hole-expanding ratio, a test was conducted inaccordance with the evaluation method of stretch-flangeability definedby the Japan Iron and Steel Federation Standard (JFST 1001-1996), todetermine the value thereof.

[0055] Values to be targeted according to the present invention are asfollows:

TS≧780 MPa; El≧18%; and λ≧60%.

[0056] The results are shown in Table 2.

[0057] It is found that each of steel sheet Nos. 2, 3, 4, 9, and 10which are examples according to the present invention satisfies therelations: TS≧780 MPa; El≧18%; and λ≧60%, and thus has high strength,and is excellent in ductility and retch-flangeability.

[0058] On the other hand, as comparative examples, steel sheet No. 1 islow in TS, due to small amount of C; steel sheet No. 5 is remarkably lowin λ, due to large amount of C and small amount of Mn; steel sheet No. 6is low in λ, due to small amount of Si; steel sheet No. 7 is low in TSand λ, due to small amount of Mn; and steel sheet No. 8 is low in El,due to large amount Mn. TABLE 1 Chemical compositions (mass %) SteelSol. No. C Si Mn P S Al N B Cr Mo Ti Nb V Zr Remark 1 0.032 1.1 2.30.012 0.004 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001<0.001 Comparative Example 2 0.054 1.0 2.3 0.015 0.002 0.030 0.003<0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Present Invention 30.065 1.4 2.1 0.010 0.003 0.030 0.003 <0.0001 <0.001 <0.001 <0.001<0.001 <0.001 <0.001 Present Invention 4 0.081 0.8 2.0 0.006 0.001 0.0300.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 PresentInvention 5 0.112 0.9 1.3 0.008 0.007 0.030 0.003 <0.0001 <0.001 <0.001<0.001 <0.001 <0.001 <0.001 Comparative Example 6 0.062 0.03 2.1 0.0140.006 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001Comparative Example 7 0.068 0.9 1.5 0.012 0.003 0.030 0.003 <0.0001<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Comparative Example 8 0.0451.2 3.6 0.010 0.002 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001<0.001 <0.001 Comparative Example 9 0.058 0.9 1.9 0.010 0.001 0.0300.003 0.0010 0.020 <0.001 <0.001 <0.001 <0.001 <0.001 Present Invention10 0.045 0.8 2.0 0.010 0.003 0.030 0.003 <0.0001 <0.001 <0.001 0.02 0.02<0.001 <0.001 Present Invention

[0059] TABLE 2 Steel Martensite Tensile properties Hole- sheet Steelvolume YP TS expanding ratio No. No. fraction (%) (MPa) (MPa) El (%) λ(%) Remark 1 1 33 408 680 27.9 85 Comparative Example 2 2 42 498 83022.9 88 Present Invention 3 3 38 510 850 22.4 80 Present Invention 4 435 630 1050 18.1 60 Present Invention 5 5 25 492 820 23.2 30 ComparativeExample 6 6 33 486 810 23.5 55 Comparative Example 7 7 26 432 720 26.440 Comparative Example 8 8 65 612 1020 13.2 85 Comparative Example 9 944 516 860 22.1 83 Present Invention 10 10 36 480 800 23.8 90 PresentInvention

Example 2

[0060] By using the slab of steel No. 2 as shown in Table 1, the stepsup to cold rolling were performed under same conditions as in Example 1and, then, continuous annealing and tempering treatment were performedunder conditions as shown in table 3. Final, temper rolling of 0.3% wasperformed in the same manner as in Example 1 to produce steel sheet Nos.A to H. Thereafter, in regard to steel sheet Nos. A to H, the same testsas in Example 1 were conducted.

[0061] The results are shown in Table 4.

[0062] It is found that each of steel sheet Nos. A, E, G, and H whichare examples according to the present invention satisfies the relations:TS≧780 MPa; El≧18%; and λ≧60%, and thus has high strength, and isexcellent in ductility and stretch-flangeability.

[0063] On the other hand, as comparative examples, steel sheet No. B islow in TS and λ, due to high heating temperature: this is considered tobe caused by that structure having martensite phase as a main componenthas become coarse; steel sheet No. C is low in TS and λ, due to shortheating time: this is considered to be caused by that a sufficientamount of austenite phase was not generated during heating and, afterrapid cooling, a sufficient volume fraction of martensite phase was notobtained; steel sheet No. D is low in TS and λ, due to low slow coolingterminal temperature: this is considered to be caused by that ferritephase was generated during the slow cooling and, after rapid cooling, avolume fraction of martensite phase was reduced; and steel sheet No. Fis low in TS and λ, due to low rapid cooling speed and high rapidcooling terminal temperature. TABLE 3 Rapid Slow cooling cooling SteelHeating Heating terminal Rapid terminal Tempering sheet Steeltemperature time Slow cooling temperature cooling rate temperaturetemperature Tempering No. No. (° C.) (sec) rate (° C./sec) (° C.) (°C./sec) (° C.) (° C.) time (sec) Remark A 2 830 150 5.0 680 2000 40 — —Present Invention B 2 890 200 5.7 720 2000 40 — — Comparative Example C2 830 5 4.7 690 2000 40 — — Comparative Example D 2 830 120 10.0 5302000 40 — — Comparative Example E 2 830 300 6.0 650 300 200 — — PresentInvention F 2 840 160 3.8 725 30 400 — — Comparative Example G 2 850 605.7 680 2000 40 200 15 Present Invention H 2 830 150 5.0 680 2000 40 30015 Present Invention

[0064] TABLE 4 Marten- Hole- site expand- Steel volume Tensileproperties ing sheet fraction YP TS ratio No. (%) (MPa) (MPa) El (%) λ(%) Remark A 39 492 820 23.2 83 Present Invention B 29 450 750 25.3 30Comparative Example C 25 438 730 26.0 45 Comparative Example D 24 432720 26.4 50 Comparative Example E 44 510 850 22.4 99 Present Invention F20 390 650 29.2 55 Comparative Example G 39 516 860 22.1 85 PresentInvention H 42 504 840 22.6 92 Present Invention

Example 3

[0065] Steel Nos. 1 to 9 having respective compositions as shown inTable 5 were each cast in to slab. The cast slab was subjected, underthe same conditions as in Example 1, to hot rolling, cold rolling,continuous annealing, and temper rolling to produce steel sheet Nos. 1to 9. Thereafter, yield strength (YP), tensile strength (TS), elongation(El), and hole-expanding ratio (k) were measured in the same manner asin Example 1.

[0066] The results are shown in Table 6.

[0067] It is found that each of steel sheet Nos. 1, 2, 3, 8, and 9 whichare examples according to the present invention satisfies the relations:TS≧780 MPa; El≧18%; and λ≧60%, and thus has high strength, and isexcellent in ductility and stretch-flangeability.

[0068] On the other hand, as comparative examples, steel sheet No. 4 islow in El and λ, due to large amount of C; steel sheet No. 5 isremarkably low in λ, due to large amount of C and small amount of Mn;steel sheet No. 6 is low in λ, due to small amount of Si; and steelsheet No. 7 is low in El and λ, due to large amount of Mn. TABLE 5Chemical compositions (mass %) Steel Sol. No. C Si Mn P S Al N B Cr MoTi Nb V Zr Remark 1 0.065 1.1 2.3 0.012 0.004 0.030 0.003 <0.0001 <0.001<0.001 <0.001 <0.001 <0.001 <0.001 Present Invention 2 0.073 1.0 2.30.015 0.002 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001<0.001 Present Invention 3 0.095 1.4 2.1 0.010 0.003 0.030 0.003 <0.0001<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Present Invention 4 0.112 0.82.0 0.006 0.001 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001<0.001 Comparative Example 5 0.134 0.9 1.3 0.008 0.007 0.030 0.003<0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Comparative Example 60.081 0.03 2.1 0.014 0.006 0.030 0.003 <0.0001 <0.001 <0.001 <0.001<0.001 <0.001 <0.001 Comparative Example 7 0.078 1.2 3.6 0.010 0.0020.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001Comparative Example 8 0.083 0.9 1.9 0.010 0.001 0.030 0.003 0.0010 0.020<0.001 <0.001 <0.001 <0.001 <0.001 Present Invention 9 0.088 0.8 2.00.010 0.003 0.030 0.003 <0.0001 <0.001 <0.001 0.02 0.02 <0.001 <0.001Present Invention

[0069] TABLE 6 Steel Martensite Tensile properties Hole- sheet Steelvolume fraction YP TS expanding No. No. (%) (MPa) (MPa) El (%) ratio λ(%) Remark 1 1 50 696 870 21.8 61 Present Invention 2 2 55 808 1010 18.870 Present Invention 3 3 51 816 1020 18.6 65 Present Invention 4 4 561000 1250 15.2 35 Comparative Example 5 5 32 792 990 19.2 30 ComparativeExample 6 6 46 744 930 20.4 45 Comparative Example 7 7 80 1024 1280 13.255 Comparative Example 8 8 47 808 1010 18.8 73 Present Invention 9 9 53800 1000 19.0 71 Present Invention

Example 4

[0070] By using the slab of steel No. 3 as shown in Table 5, the stepsup to cold rolling were performed under the same conditions as inExample 1 and, then, continuous annealing and tempering treatment wereperformed under conditions as shown in Table 7. Finally, temper rollingof 0.3% was performed in the same manner as in Example 1 to producesteel sheet Nos. A to J. Thereafter, in regard to steel sheet Nos. A toJ, the same tests as in Example 1 were conducted.

[0071] The results are shown in Table 8.

[0072] It is found that each of steel sheet Nos. B, G, I, and J whichare examples according to the present invention satisfies the relations:TS≧780 MPa; El≧18%; and λ≧60%, and thus has high strength, and isexcellent in ductility and stretch-flangeability.

[0073] On the other hand, as comparative examples, steel sheet No. A islow in El, due to low heating temperature; steel sheet No. C is low inλ, due to high heating temperature: this is considered to be caused bythat structure having martensite phase as a main component has becomecoarse; steel sheet No. D is low in λ, due to short heating time: thisis considered to be caused by that austenite phase was not sufficientlygenerated during heating and, after rapid cooling, a sufficient volumefraction of martensite phase was not obtained; steel sheet No. E is lowin El, due to high slow cooling terminal temperature; steel sheet No. Fis low in TS and λ, due to low slow cooling terminal temperature: thisis considered to be caused by that ferrite phase was generated duringthe slow cooling and, after rapid cooling, a volume fraction ofmartensite phase was reduced; and steel sheet No. F is low in TS and λ,due to low rapid cooling rate and high rapid cooling terminaltemperature. TABLE 7 Rapid Slow cooling cooling Steel Heating Heatingterminal Rapid terminal Tempering sheet Steel temperature time Slowcooling temperature cooling rate temperature temperature Tempering No.No. (° C.) (sec) rate (° C./sec) (° C.) (° C./sec) (° C.) (° C.) time(sec) Remark A 3 740 300 3.0 650 2000 40 — — Comparative Example B 3 830150 5.0 680 2000 40 — — Present Invention C 3 890 200 5.7 720 2000 40 —— Comparative Example D 3 830 5 4.7 690 2000 40 — — Comparative ExampleE 3 830 270 1.7 780 2000 40 — — Comparative Example F 3 830 120 10.0 5302000 40 — — Comparative Example G 3 830 300 6.0 650 300 200 — — PresentInvention H 3 840 160 3.8 725 30 400 — — Comparative Example I 3 850 605.7 680 2000 40 200 15 Present Invention J 3 830 150 5.0 680 2000 40 30015 Present Invention

[0074] TABLE 8 Steel Martensite Tensile properties sheet volume fractionYP TS Hole-expanding No. (%) (MPa) (MPa) El (%) ratio λ (%) Remark A 651024 1280 14.8 65 Comparative Example B 55 840 1050 18.1 68 PresentInvention C 41 712 890 21.3 30 Comparative Example D 43 760 950 20.0 45Comparative Example E 62 960 1200 14.0 71 Comparative Example F 24 576720 26.4 50 Comparative Example G 53 800 1000 19.0 61 Present InventionH 20 616 770 24.7 42 Comparative Example I 52 824 1030 18.4 62 PresentInvention J 56 808 1010 18.8 68 Present Invention

1. A high strength cold-rolled steel sheet consisting essentially of, interms of percentages by mass, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3%Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% orless N, optionally at least one element selected from the groupconsisting of Cr, Mo, B, Ti, Nb, V and Zr, and the balance being ironand inevitable impurities, and having a structure substantiallycomprising a ferrite phase and a martensite phase.
 2. The high strengthcold-rolled steel sheet as set forth in claim 1, wherein, in terms ofpercentages by mass, C is in an amount of 0.04 to less than 0.070%, andsaid steel sheet has a tensile strength of from 780 MPa to less than 980MPa.
 3. The high strength cold-rolled steel sheet as set forth in claim1, wherein, in terms of percentages by mass, C is in an amount of 0.070to 0.10%, and said steel sheet has a tensile strength of from 980 MPa toless than 1180 MPa.
 4. The high strength cold-rolled steel sheet as setforth in claim 1, further containing at least one element selected fromthe group consisting of, in terms of percentages by mass, 0.01 to 1.0%Cr, 0.01 to 0.5% Mo, 0.0001 to 0.0020% B, 0.001 to 0.05% Ti, 0.001 to0.05% Nb, 0.001 to 0.05% V and 0.001 to 0.05% Zr.
 5. The high strengthcold-rolled steel sheet as set forth in claim 2, further containing atleast one element selected from the group consisting of, in terms ofpercentages by mass, 0.01 to 1.0% Cr, 0.01 to 0.5% Mo, 0.0001 to 0.0020%a, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001 to 0.05% and 0.001 to0.05% Zr.
 6. The high strength cold-rolled steel sheet as set forth inclaim 3, further containing at least one element selected from the groupconsisting of, in terms of percentages by mass, 0.01 to 1.0% Cr, 0.01 to0.5% Mo, 0.0001 to 0.0020% B, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb,0.001 to 0.05% V and 0.001 to 0.05% Zr.
 7. The high strength cold-rolledsteel sheet as set forth in claim 2, wherein said steel sheet has amartensite phase in a volume fraction of 30% to 45%.
 8. The highstrength cold-rolled steel sheet as set forth in claim 5, wherein saidsteel sheet has a martensite phase in a volume fraction of 30% to 45%.9. The high strength cold-rolled steel sheet as set forth in claim 3,wherein said steel sheet has a martensite phase in a volume fraction of45% to 60%.
 10. The high strength cold-rolled steel sheet as set forthin claim 6, wherein said steel sheet has a martensite phase in a volumefraction of 45% to 60%.
 11. A method for producing a high strengthcold-rolled steel sheet comprising the steps of: (a) producing a steelsheet by hot rolling a steel slab comprising a composition as set forthin any one of claims 1 to 6, followed by cold rolling to provide acold-rolled steel sheet; (b) heating the cold-rolled steel sheet at from750° C. to 870° C. for 10 seconds or more to provide a heated steelsheet; (c) cooling the heated steel sheet down to from 550° C. to 750°C. to provide a cooled steel sheet; and (d) cooling the cooled steelsheet down to 300° C. or less at a cooling rate of over 100° C./sec. 12.The method for producing a high strength cold-rolled steel sheet as setforth in claim 11, wherein the steel sheet is cooled at a cooling rateof 20° C./sec. or less within a temperature range of from 550° C. to750° C.
 13. The high strength cold-rolled steel sheet as set forth inclaim 1, wherein, in terms of percentages by mass, the Si is in anamount of 0.5 to 1.0% and the Mn is in an amount of 2.0 to 2.5%.
 14. Thehigh strength cold rolled steel sheet as set forth in claim 13, furthercontaining at least one element selected from the group consisting of,in terms of percentages by mass, 0.01 to 1.0% Cr, 0.01 to 0.5% Mo,0.0001 to 0.0020% B, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001 to0.05% V and 0.001 to 0.05% Zr.
 15. The high strength cold-rolled steelsheet as set forth in claim 1, wherein, in terms of percentages by mass,C is 0.054%, Si is 1.0%, Mn is 2.3%, P is 0.015%, S is 0.002%, Sol.Al is0.030%, and N is 0.003%.
 16. The high strength cold-rolled steel sheetas set forth in claim 1, wherein, in terms of percentages by mass, C is0.065%, Si is 1.4%, Mn is 2.1%, P is 0.010%, S is 0.003%, Sol.Al is0.030%, and N is 0.003%.
 17. The high strength cold-rolled steel sheetas set forth in claim 1, wherein, in terms of percentages by mass, C is0.081%, Si is 0.8%, Mn is 2.0%, P is 0.006%, S is 0.001%, Sol.Al is0.030%, and N is 0.003%.
 18. The high strength cold rolled steel sheetas set forth in claim 1, wherein, in terms of percentages by mass, C is0.058%, Si is 0.9%, Mn is 1.9%, P is 0.010%, S is 0.001%, Sol.Al is0.030%, N is 0.003%, B is 0.0010%, and Cr is 0.020%.
 19. The highstrength cold rolled steel sheet as set forth in claim 1, wherein, interms of percentages by mass, C is 0.045%, Si is 0.6%, Mn is 2.0%, P is0.010%, S is 0.003%, Sol.Al is 0.030%, N is 0.003%, Ti is 0.02%, and Nbis 0.02%.
 20. The high strength cold-rolled steel sheet as set forth inclaim 1, wherein, in terms of percentages by mass, C is 0.065%, Si is1.1%, Mn is 2.3%, P is 0.012%, S is 0.004%, Sol.Al is 0.030%, and N is0.003%.
 21. The high strength cold-rolled steel sheet as set forth inclaim 1, wherein, in terms of percentages by mass, C is 0.073%, Si is1.0%, Mn is 2.3%, P is 0.015%, S is 0.002%, Sol.Al is 0.030%, and N is0.003%.
 22. The high strength cold-rolled steel sheet as set forth inclaim 1, wherein, in terms of percentages by mass, C is 0.095%, Si is1.4%, Mn is 2.1%, P is 0.010%, S is 0.003%, Sol.Al is 0.030%, and N is0.003%.
 23. The high strength cold-rolled steel sheet as set forth inclaim 1, wherein, in terms of percentages by mass, C is 0.083%, Si is0.9%, Mn is 1.9%, P is 0.010%, S is 0.001%, Sol.Al is 0.030%, N is0.003%, B is 0.0010%, and Cr is 0.020%.
 24. The high strengthcold-rolled steel sheet as set forth in claim 1, wherein, in terms ofpercentages by mass, C is 0.088%, Si is 0.8%, Mn is 2.0%, P is 0.010%, Sis 0.003%, Sol.Al is 0.030%, N is 0.003%, Ti is 0.02%, and Nb is 0.02%.25. The method for producing a high strength cold-rolled steel sheet asset forth in claim 11, wherein the steel sheet is cooled down to 100° C.or less at a cooling rate of 500° C./sec or more in step (d).
 26. Themethod for producing a high strength cold-rolled steel sheet as setforth in claim 11, further comprising a step of tempering the cooledsteel sheet at from 150 to 390° C. for 5 to 20 minutes after step (d).